Thermoset plastics are favored for making many kinds of fiber-reinforced articles because of their ease of manufacture. Uncured thermosets are often low viscosity liquids at room temperature and easily wet a fabric of fibers. Once they have migrated through the fabric and surrounded its fibers, a curing stage (sometimes called a hardening stage) commences to polymerize the thermoset into a polymer matrix. Often, this wetting and curing takes place in a mold that defines the shape of the fiber-reinforced article.
The uncured thermoset resins used to make the composite are generally inexpensive, but often off-gas irritating and sometimes dangerous volatile organic compounds (VOCs). The outgassing of VOCs are of particular concern during curing, when the exothermic nature of many thermoset curing reactions raise the temperature of the composite and drive more VOCs into the gas phase. In many instances, it is necessary to cure large thermoset articles in facilities equipped with robust ventilation and air scrubbing equipment, increasing the overall production costs.
Thermoset articles are also difficult to repair or recycle. Hardened thermoset resins often have a high degree of crosslinking, making them prone to fractures and breaks. Because thermosets normally will not melt under heat, they have to be replaced instead of repaired by welding. Compounding difficulties, the unrepairable thermoset part normally cannot be recycled into new articles, but must instead be landfilled at significant cost and adverse impact on the environment. The problems are particularly acute when large thermoset parts, such as automotive panels and wind turbine blades, need to be replaced.
Because of these and other difficulties, thermoplastic resin systems are being developed for fiber-reinforced articles that were once exclusively made using thermosets. Thermoplastics typically have higher fracture toughness and chemical resistance than thermosets. They also melt at raised temperatures, allowing operators to heal cracks and weld together pieces instead of having to replace a damaged part. Perhaps most significantly, discarded thermoplastic parts can be broken down and recycled into new articles, reducing landfill costs and stress on the environment.
Unfortunately, thermoplastic composites have their own challenges. High melt viscosities of thermoplastic polymer resins may cause difficulties in impregnating reinforcing fibers. Conventional techniques for producing thermoplastic composites, such as extrusion compounding, break fibers down to very short lengths, which limits mechanical properties of composite articles. Existing processes to produce thermoplastic composites containing long or continuous fibers often result in incomplete resin impregnation and poor bonding between thermoplastic matrix and reinforcing fibers. Thus, there is a need to develop new ways to improve mechanical properties of the thermoplastic composite materials. These and other issues are addressed in the present application.